Sialic acid is a sugar with a net negative charge. It is often present on terminating branches of N-glycans, O-glycans, and glycosphingolipids (gangliosides), and occasionally capping side chains of GPI anchors. Sialic acid modification of cell surface molecules plays a role in many biological phenomena such as protein structure stability, regulation of cell adhesion, and signal transduction. Sialic acid deficiency disorders such as Hereditary Inclusion Body Myopathy (HIBM or HIBM type 2), Nonaka myopathy, and Distal Myopathy with Rimmed Vacuoles (DMRV) are clinical diseases resulting from a reduction in sialic acid production.
HIBM is a rare autosomal recessive neuromuscular disorder caused by a biosynthetic defect in the sialic acid synthesis pathway. Eisenberg et al., Nat. Genet. 29:83-87 (2001). The disease usually manifests between the ages of 20 to 40 such as foot drop and slowly progressive muscle weakness and atrophy. Patients may suffer difficulties walking with foot drop, gripping, use of hands, and swallowing. The disease is progressive; most afflicted individuals become incapacitated and wheelchair-confined within two to three decades. No treatments are available.
Studies of an Iranian-Jewish genetic isolate mapped the mutation associated with HIBM to chromosome 9p12-13. Argov et al., Neurology 60:1519-1523 (2003). The causative mutations were identified for HIBM in the gene GNE, which encodes the bifunctional enzyme UDP-N-acetylglucosamine-2-epimerase/N-acetylmannosamine kinase (GNE/MNK). Eisenberg et al., Nat. Genet. 29:83-87 (2001). DMRV is a Japanese variant, allelic to HIBM. Nishino et al., Neurology 59:1689-1693 (2002).
The biosynthesis steps and feedback regulation of GNE/MNK is depicted in FIG. 1. The production of sialic acid on glycoconjugates requires the conversion of N-acetylglucosamine (conjugated to its carrier nucleotide sugar UDP) to sialic acid. The sialic acid subsequently enters the nucleus where it is conjugated with its nucleotide sugar carrier CMP to make CMP-sialic acid, which is used as a donor sugar for glycosylation reactions in the cell. CMP-sialic acid is a known regulator of GNE/MNK activity. Jay et al., Gene Reg. & Sys. Biol. 3:181-190 (2009). Patients with HIBM have a deficiency in the production of sialic acid by the GNE/MNK enzyme, which is involved in the first two steps of this sequence. Nearly twenty GNE mutations have been reported in HIBM patients from different ethnic backgrounds with founder effects among the Iranian Jews and Japanese. Broccolini et al., Hum. Mutat. 23:632 (2004).
Because the production of sialic acid is the key reason the mutation causes the disease, replacing a metabolite after the genetic block in the pathway could, in theory, alleviate symptoms of a sialic acid deficiency. Jay et al., Gene Reg. and Sys. Biology 3:181-190 (2009). In practice, however, administering one or more compounds in the sialic acid biosynthetic pathway in vivo is a significant challenge. These compounds have extraordinarily rapid clearance rates and are excreted in the urine before they can be metabolized.